Substantial research has been directed toward new battery systems which are lightweight, long lived, low volume, and require little or no maintenance. In particular, development of new rechargeable, electric batteries has been an area of active research.
The nickel-hydrogen battery is one such electric storage system which offers many advantages including high specific energy, long cycle life, state-of-charge indication, and tolerance of overcharge, overdischarge and reversal. Development of this battery has been mainly directed toward aerospace applications. However, research programs are now active in developing the battery for use in terrestrial applications, although these batteries are not yet commercially available. While cost has generally not been a limiting factor in aerospace developments, on a cost per cycle basis, nickel-hydrogen batteries have been found to be competitive with other rechargeable systems.
Batteries or cells are devices that convert chemical energy contained in materials directly into electrical energy by means of an electrochemical oxidation-reduction reaction. This type of reaction involves the transfer of electrons from one material to another through an electrical circuit. The basic electrochemical unit is a "cell". A "battery" consists of one or more cells, connected in series or parallel, or both, depending on the desired output voltage and capacity.
A basic cell unit consists of three major components: (1) an anode, or negative electrode, which gives up electrons to the external circuit and is oxidized during the electrochemical reaction; (2) a cathode, or positive electrode, which accepts electrons from the external circuit and is reduced during the electrochemical reaction; and (3) an electrolyte, or ionic conductor, which provides the medium for transfer of electrons, as ions, inside the cell between the anode and the cathode. The electrolyte is typically a liquid with dissolved salts, bases or acids to impart ionic conductivity. Some cells, however, use solid electrolytes.
Physically, the anode and cathode electrodes are isolated in the cell to prevent internal short-circuiting but are surrounded by the electrolyte. A separator material is typically used to mechanically separate the anode and cathode. The separator is permeable to electrolyte in order to maintain the desired ionic conductivity.
Secondary batteries are rechargeable, that is, able to go through a series of discharge-charge cycles. In these batteries, electrons flow from the anode to cathode on discharge. On recharge, electrons are delivered to the anode so that the anode and/or cathode are restored to their original form. Secondary batteries generally have good energy densities, high discharge rates and good low temperature performance. Their energy densities are generally lower than those of primary, i.e., non-rechargeable, batteries. Their charge retention on standing is also poorer than most primary batteries.
The most advantageous combinations of anode and cathode materials are those that will be the lightest in weight and give high cell voltage and capacity. Such combinations may not always be practical, however, due to factors such as reactivity with other cell components, difficulty in handling and high cost. In a practical design, the anode selected operates efficiently, has good conductivity, is stable, and is easily fabricated at low cost. The cathode must also operate efficiently and be stable when in contact with the electrolyte.
The major components of a nickel-hydrogen cell are a nickel hydroxide electrode, a hydrogen catalytic electrode, a separator, an electrolyte, typically an alkali-metal hydroxide, such as potassium hydroxide, and hydrogen gas. A gas-diffusion or gas-permeable plate or screen is typically included to facilitate hydrogen diffusion into the back of the hydrogen electrode. The components of one or more cells are housed within a vessel so designed to contain the hydrogen gas under pressure. A multiple of cells would be connected electrically either in series or in parallel.
The nickel hydroxide electrode of a nickel-hydrogen cell is usually designated the "cathode" or positive electrode, and the hydrogen catalytic electrode is usually designated the "anode" or negative electrode. During discharge, when the cell is connected to an external load, electrons flow from the anode or negative electrode, i.e., the hydrogen electrode, where hydrogen is oxidized to water, through the external load to the cathode or positive electrode, i.e., the nickel electrode, where nickel oxyhydroxide is reduced to nickel hydroxide. During charge, the current flow is reversed and oxidation takes place at the nickel electrode and reduction at the hydrogen electrode, i.e., the nickel hydroxide is oxidized to nickel oxyhydroxide and hydrogen gas is reformed from reduction of water. Thus, after recharge, the battery reverts to its original chemical state.
The positive electrode is typically comprised of electroactive material on a conductive medium. The negative electrode is generally a plastic bonded, metal powder plate in which the metal, for example, platinum, will catalyze hydrogen oxidation in an aqueous medium. The catalytic electrode theoretically undergoes no physical or chemical changes itself, and is, thus, very stable and depth-of-discharge has virtually no effect on it. This electrode is often constructed with an electrolyte wettable side and an electrolyte nonwettable side to facilitate hydrogen gas access. The wettable side faces the separator and the nonwettable side faces the gas diffusion screen.
The separator is sufficiently thick to prevent short circuit contact between the electrodes yet sufficiently absorbent to hold an appropriate quantity of electrolyte to allow the electrochemical reactions to occur in the cell. The electrolyte is primarily held within the separator and the electrodes.
Various prior art nickel-hydrogen cell stack designs are known. The term "cell stack" refers to an assembly of a one or more basic cell units. The term "cell stack configuration" refers to the positioning of the basic cell components with respect to one another. Such positioning generally relates to controlling various supporting features of cell design, such as oxygen and electrolyte management, as will be explained below.
One type of cell stack configuration is the so-called back-to-back arrangement in which two basic cell units are paired with the positive or negative electrodes back-to-back. In a back-to-back, positive electrode configuration, for example, the cell stack for the cell-unit pair consists of, in consecutive order, a first diffusion screen, a first negative electrode, a first separator, the two positive electrodes, a second separator, a second negative electrode and a second diffusion screen. See, for example, Van Ommering, et al., U.S. Pat. No. 4,115,630, issued Sept. 19, 1978; Holleck, U.S. Pat. No. 4,127,703, issued Nov. 28, 1978; Warnock, U.S. Pat. No. 4,038,461, issued July 26, 1977.
Another cell stack design uses only one positive electrode with two negative electrodes positioned on either side and separated by a separator. See, for example, Dunlop et al., U.S. Pat. No. 3,867,199, issued Feb. 18, 1975; Plust et al., U.S. Pat. No. 4,117,206, issued Sept. 26, 1978; Gutmann et al., U.S. Pat. No. 4,215,184, issued July 29, 1980.
Other nickel-hydrogen cell designs have simply used alternating positive electrode-negative electrode couples, with a separator between each positive and negative electrode. See, for example, Tsenter et al., U.S. Pat. No. 3,669,744, issued June 13, 1972.
Various problems, however, have been encountered in the design and structure of nickel-hydrogen cells and batteries, particularly in proposed aerospace applications where available space is minimal, making high energy density a necessity. The present limitations of the nickel-hydrogen cells and batteries are due primarily to deficiencies in the supporting aspects of cell design rather than, for example, in electrode technology. Limitations can be cured by developing cell design features which are not in themselves life limiting.
A persistent and vexatious problem, largely unattended by the prior art, and caused to some extent by the constraints imposed by the aerospace applications, is the lack of proper oxygen and electrolyte management. Evolution of oxygen near the end of charge and during overcharge can produce undesirable effects within the cells and can impose undesirable constraints in the design and operation of cells. Oxygen is evolved at the cathode during overcharge because the cathode reaches an end point corresponding to the fully oxidized state of the nickel hydroxide. At the same time, the anode has no such end point since it continues to consume water to produce hydrogen gas. Since no effective method has been found to eliminate oxygen evolution completely, various means of recombining the oxygen with hydrogen to form water with minimal disruption of cell operation have been sought. Ideally, the rate of oxygen recombination should equal the rate at which oxygen is generated. Oxygen produced within a cell should be recombined to form water within the same cell.
Generally, past aerospace cells had uncontrolled oxygen recombination. Pockets of oxygen built up, resulting in rapid recombination with hydrogen, which caused burn holes to appear in the negative electrode and melting of plastic separators, if such were used, because of the locally excessive heat released.
Oxygen management can also be compromised by separator swelling when fuel cell grade asbestos is used as the separator material. In order to achieve the long cycle life of which a nickel-hydrogen cell or battery is capable, use of surplus electrolyte is necessary because morphology changes in the nickel electrode result in increased propensity of this electrode to hold electrolyte as a function of cycling. The separator border, however, often swells from soaking up the excess electrolyte, sometimes three or four times its original thickness, blocking orderly migration of oxygen to the negative electrode where it can be converted to water.
Electrolyte management is also essential in a nickel-hydrogen cell design because so many factors work against it. The storage of hydrogen in the pressure vessel provides a volume into which electrolyte can escape. Cell operation tends to force electrolyte out of the electrode stack mainly by displacement, but also by entrainment. As oxygen bubbles form in the cathode and move, they push electrolyte out of the cathode and the separator. Moving gases, such as the bubbles, may also entrain electrolyte. When oxygen bubbles break at the perimeter of the cathode, a fine mist of entrained electrolyte is released to the hydrogen gas space of the cell. Loss of electrolyte results in progressive reduction of accessible electrode active material and of the ion conducting pathways between anode and cathode.
Electrolyte concentration gradients can result from uneven production of water, e.g., when oxygen recombination is unevenly distributed over the surface of the hydrogen catalytic electrode. Electrolyte gradients can cause current density gradients and related imbalances in cell operation which degrade performance and life. Water in the electrolyte can also be lost by evaporation from a hot electrode stack and condensed on the cool interior walls of the pressure vessel.
Yet another electrolyte management problem results from use of certain types of diffusion screens. The diffusion screens of the prior art were normally a woven material. The woven screens have a propensity to trap the potassium hydroxide electrolyte during cycle testing of the cell and if charged with the electrolyte under vacuum. Electrolyte so trapped in the screens reduces gas access to the negative electrodes, and, hence, reduces the cell performance.
Another deficiency in the supporting aspects of cell design involves mechanical integrity. Typically, many basic cell units must be stacked to achieve the desired capacity of a cell. This requires proper physical alignment of a large number of cell components. Separators must physically isolate positive and negative electrodes. Diffusion screens must be positioned on the surface of the negative electrode to facilitate hydrogen gas access. Often, for example, screens slip against the polytetrafluorothylene used as the nonwettable surface of the negative electrode.
Various prior art cells have attempted to control some of the oxygen, electrolyte and mechanical problems explained above by modifying the cell stack design, or by changing the the physical properties or size of cell components.
One nickel-hydrogen cell, which uses a back-to-back positive electrode arrangement, utilizes two electrolyte matrices, each backed with a microporous hydrophobic membrane, between the back-to-back electrodes. The matrices, which consist of a nonwoven polymeric fabric, attempt to manage the electrolyte by taking up electrolyte displaced from its normal location in and between the electrodes by evolved gases and returning it to its normal location. The membrane, which consists of porous polymer, permits gas and vapor therethrough while being impermeable to the liquid electrolyte. See, Holleck, U.S. Pat. No. 4,127,703, issued Nov. 28, 1978.
In another cell design, electrolyte reservoirs positioned either against the walls of the pressure vessel or walls of a cell case are employed as wicks for electrolyte. See Warnock, "Design of Nickel-Hydrogen Cells for Spacecraft", Proceedings of Symposium on Battery Design and Optimization, S. Gross, ed., The Electrochemical Society (1979), pp. 163-178; Holleck, U.S. Pat. No. 4,327,158, issued, Apr. 27, 1982.
Yet another nickel-hydrogen cell discloses an electrode stack arrangement as the recirculating design in which the positive and negative electrodes are stacked alternately with the nickel electrode of one cell unit facing the hydrogen catalytic electrode of the next. This arrangement allows for oxygen generated in the cathode of one cell unit to recombine over the entire surface of the hydrogen electrode of the next. This cell also provides a conduit external to the cell stack for returning the oxygen at one end of the stack to the other end for oxygen-hydrogen recombination. This oxygen return scheme prevents asymmetric buildup of water in cell stack. See, Warnock, U.S. Pat. No. 4,038,461, issued July 26, 1977.
Various techniques have also been used to insure proper alignment of all cell components and prevent swelling of components. The so-called "pineapple slice" configuration utilizes cell components, including two stack end plates, which are annular and have a central aperture through which a rod or screw is placed and bolted at either or both ends to hold the stack fast See, for example, Warnock, U.S. Pat. No. 3,955,210, issued Aug. 17, 1976; Plust et al., U.S. Pat. No. 4,117,206, issued Sept. 26, 1978. Another design uses bolts and end plates but the apertures are not centrally located. See, for example, Dunlop et al., U.S. Pat. No. 3,867,199, issued Feb. 18, 1975.
In the assembly of cells into batteries, yet another electrolyte management problem is electrolyte bridging between the cells of the battery. Electrolyte bridging between adjacent cells results in undesirable parasitic shunt currents within the battery. Parasitic shunt currents limit the performance, especially long life, of a battery.
One prior art battery, which utilizes the positive electrodes in a back-to-back configuration, uses a cell case to separate cell pairs or stacks from each other. The cell separator is hydrophobic and prevents electrolyte bridging between adjacent modules because it is nonporous in the direction perpendicular to the electrode face. The cell case or envelope may also contain scratches or grooves or a thin plastic screen on each surface to provide a gas transport layer to facilitate gas access to the surfaces of the negative electrodes. See, Van Ommering, U.S. Pat. No. 4,115,630, issued Sept. 19, 1978.
A further improvement on the above battery involves using a plurality of electrode stacks encased in hydrophobic cups having a gap between them. The gap is sufficient to allow gas stored within the pressure vessel to enter each stack, but also to inhibit electrolyte bridging. See, Holleck, U.S. Pat. No. 4,327,158, issued Apr. 27, 1982.
Another battery utilizes a porous material, with a wettable surface between negative electrodes which are positioned back-to-back. The cell unit pairs are contained within a plastic cell case. The case is made of a nonwetting material and constructed to permit hydrogen gas flow into the case but to restrict electrolyte from leaving. See, Warnock, U.S. Pat. No. 3,975,210, issued Aug. 17, 1976.
Despite recognition of practical cell and battery design problems, proper solution to all these problems in a single cell or battery design has not been demonstrated in the prior art. The present invention provides a nickel-hydrogen cell and battery with improved and cooperative oxygen and electrolyte management and component alignment features.